1. Field of the Invention
This invention is related to the general field of lenses with variable focal length. In particular, the invention consists of devices with variable optical properties changed by the introduction of very thin liquid layers between opposite optical surfaces.
2. Description of the Prior Art
Presbyopia, disease, genetic conditions and accidents often affect people's ability to focus over a wide range of viewing distances. Therefore, most people at some point in their lives need to wear glasses to correct their eyesight, often with different correction requirements for distant and near vision. To that end, people use bifocal or even trifocal lenses that provide specific correction over predetermined distances, but these lenses do not afford good correction for intermediate distances. Therefore, the ideal corrective lens should have adjustable refractive power to provide variable vision compensation for any distance.
Accordingly, for many years inventors have been trying to develop eyeglasses with controllable focusing power. Various inventions have involved the use of conventional mechanically-operated zoom lenses, which are relatively heavy, and liquid-crystal lenses, which suffer from turbidity and insufficient refractive-index change. Another approach has been to use various forms of liquid lenses, which typically have small apertures, poor optical focusing, and require large amounts of liquid to change their properties, causing them also to be heavy and cumbersome.
Several patents exist for eyeglasses with variable focal length obtained by moving solid lenses, as in zoom lenses. See, for example, U.S. Pat. No. 4,865,438. Because of the multiple lenses and mechanisms required for the variable focal length feature, these glasses are necessarily heavy and cumbersome. Thus, they are not yet found in standard eyewear.
A more common approach to achieve this goal has been through the use of liquid or deformable-gel lenses. See, for example, U.S. Pat. Nos. 2,437,642, 2,576,581, 3,161,718, 3,614,215, 3,738,734, 4,174,156, and 4,466,706. All of these patents disclose inventions designed to produce varied focal lengths by changing the content, shape or thickness of fluid-filled lenses, thus providing variable refractive characteristics in a single lens.
In particular, U.S. Pat. No. 4,181,408 discloses a vision compensation system that includes fluid lenses that are adaptively adjustable to correct for vision at different focal lengths. The refractive power of the lenses, mounted on a standard spectacle frame, is changed by varying their curvature as a function of the amount of liquid retained in them, which is in turn controlled by a hydraulic pump system that is operable either manually or automatically.
A similar device was disclosed in U.S. Pat. No. 5,182,585, involving eyeglasses equipped with liquid lenses having adjustable refractive power that depends on the amount of liquid forced into each lens. An electrically powered pump and an electronic controller regulate the liquid input to the lenses in response to a signal determined by the focal length required for viewing a given object in the field of vision, as determined by a rangefinder mounted on the frame of the eyeglasses.
Other patents involving liquid-filled lenses were issued to Croll (U.S. Pat. No. 5,013,145), describing novelty eyeglasses that use a space between lenses that can be filled with a colored liquid (but are not focusable thereby); Gerber (U.S. Pat. No. 4,418,990), disclosing glasses having variable focal length adjusted by pressure along the lens; Quaglia (U.S. Pat. No. 5,739,959), teaching the use of a clear magnetic fluid and solid optical components moved via electromagnets to automatically focus eyeglasses; and Cronin et al. (U.S. Pat. No. 5,526,067) and Kurtin et al. (U.S. Pat. No. 5,138,494), teaching the use of liquid surfaces to form lenses with variable focal length.
Obviously, the complex hardware involved in these inventions is sensitive to any change in physical properties, such as elasticity or temperature, that may cause misalignments of the various components with attendant optical defects, or require a new calibration. Moreover, major problems are related to the high mass and poor optical quality that result from having a relatively large volume of liquid contained between optical membranes.
Several patents teach the application of liquid-crystal layers for use in eyeglasses. Such layers, having optical properties that depend upon applied electric fields, can be used to change the transmissive or the focal properties of eyewear lenses according to the desires of the wearer (see, for example, U.S. Pat. No. 5,113,270).
Intraocular lenses have also been envisioned with focal properties varied by amount or type of injected fluid, or by voltages applied to liquid crystals, or by holographic fragments.
The principal difficulty in making eyeglasses having sufficient focal range with liquid crystals is the limited range of refractive index available due to change of electric fields, which requires thick layers of fluids, and the turbidity of such liquids, making the transmission of thick layers unsatisfactory for use in eyewear. Additionally, there are technical difficulties in producing the necessary precise distributions of high voltages to enable good focusing.
Lack of sufficient range of refractive index also limits the use of liquid crystals in thin layers which alternately match, or do not match, the refractive indices of the surfaces enclosing them, so that diffractive lenses present on those surfaces are optically present or absent according to voltages applied to the liquid crystals. The principal difficulty of this approach, to obtain sufficient index mismatch to enable a lens to focus a large fraction of the light passing through it, remains largely unsolved.
Therefore, this invention is directed at a novel approach to providing variable focusing power of an optical system, wherein the focal length is determined by the optical properties and variable combinations of various fixed solid components in a device, rather than the manipulation and deformation of liquid layers and flexible membranes.